Method for production of inorganic nanofibres through electrostatic spinning

ABSTRACT

The present disclosure relates to the production method of inorganic nanofibres through electrostatic spinning of solution, which comprises alkoxide of metal or of semi-metal or of non-metal dissolved in a solvent system on basis of alcohol. The solution is stabilised by chelating agent, which prevents hydrolysis of alkoxide, and after homogenisation it is mixed with solution of poly(vinylpyrrolidone) in alcohol, after then the resultant solution is brought into electrostatic field, in which the electrostatic spinning is running continually, the result of which is production of organic-inorganic nanofibres, which are after then calcinated outside the spinning device in the air atmosphere at the temperature from 500° C. to 1300° C.

TECHNICAL FIELD

The invention relates to a method for production of inorganic nanofibresthrough electrostatic spinning of solution, which contains alkoxide ofmetal or of semi-metal or of non-metal dissolved in a solvent system onbasis of alcohol.

BACKGROUND ART

Inorganic materials feature a number of properties, thanks to which theyare suitable for usage in many technical fields, e.g. inelectrotechnics, medicine, industry, etc. For example, TiO₂, SiO₂,Al₂O₃, ZrO₂ and B₂O₃ belong to the important inorganic substances. Atinorganic nanofibres there are combined properties of nanofibrousmaterials, like an organised one-dimensional structure with propertiesof nanomaterials, especially with high specific surface, and withphysical-chemical properties of inorganic substances as hardness,thermal resistance and structure of electron stripes. Therefore theresultant nanofibres are suitable for production of composite materials,catalysts, electrochemical elements, etc.

At present, there are known various methods for production ofnanoparticles from inorganic materials. Production of inorganicnanoparticles, namely from SiO₂ and Al₂O₃, is disclosed inWO2007/079841.

Nanoparticles from inorganic material, which may be produced through theabove mentioned or other suitable method, could also be incorporatedinto a structure of nanofibres, which may be realised e.g. by adding thenanoparticles into polymer solution and consequent production ofnanofibres from this solution. Presence of inorganic nanoparticles inpolymer nanofibres renders specific properties to these nanofibres.Nevertheless the substantial portion of these nanoparticles is formed ofpolymer component.

At present the pure inorganic nanofibres are produced by discontinualmethods of electrostatic spinning at usage of nozzle or needle spinningelectrode, into which the solution is supplied, which may be representedby a precursor of given inorganic elements, or the polymer solutioncontaining alkoxide of respective metal or non-metal as a source ofinorganic basis of fibres.

The known solutions used for production of anorganic nanofibres throughelectrostatic spinning from nozzles cannot be used for continualproduction of nanofibres, because alkoxides are time-unsteady and areeasily subject to degradation of alkoxide through hydrolysis, this eventhrough action of air humidity, which occurs still before theirspinning. To date for electrostatic spinning there was not used anysolution of alkoxide, that would be stable enough and could be used forcontinual production of inorganic nanofibres.

The goal of the invention is to develop a continual production method ofinorganic nanofibres through electrostatic spinning, which would removedisadvantages of the background art.

The Principle of Invention

The goal of the invention has been achieved through a method forproduction of inorganic nanofibres according to the invention, whoseprinciple consists in that, the alkoxide solution of metal or ofsemi-metal or non-metal is in a solvent system on basis of alcoholstabilised by chelating agent, which prevents hydrolysis of alkoxide,and after homogenizing is mixed with solution of poly(vinylpyrrolidone)in alcohol, after then the resultant solution is brought intoelectrostatic field, in which continually on long-term basis theelectrostatic spinning is running, the result of which is production oforganic-inorganic nanofibres, which are outside the spinning devicecalcinated after then in the air atmosphere at the temperature from 500°C. to 1300° C.

By stabilisation of solution the hydrolysis of alkoxides by action ofair humidity and other impacts of working environment is prevented, sothat the process of electrostatic spinning runs continually and in thelong-term. In the published works, in case of nozzle electro-spinning,they use the solutions of alkoxide of metal in alcohol in combinationwith poly(vinylpyrrolidone). Alkoxide is stabilised by additive ofacetic acid (see Journal of America Ceramic Society89[6]1861-1869(2006), Science and Technology of Advanced Materials6(2005)240-245). Usage of this solution in case of electrostaticspinning from opened surface is possible in laboratory scale,nevertheless at the process lasting longer than half an hour thedegradation of solution and hydrolysis of alkoxide occurs. This effectprevents industrial utilization of in literature described compositionsof solutions for production of ceramic nanofibres through the method ofelectrostatic spinning from opened surface.

For the purpose to increase the electrical conductivity of solution andto increase efficiency of production process, it is possible to add intothe solution a concentrated acid, which is according to the claim 3preferably selected from the group of hydrochloric acid, nitric acid,phosphoric acid.

In preferred embodiment of the method the chelating agent is composed ofβ-diketone, while the most suitable β-diketone seems to beacetylacetone, at whose usage the stabilisation of solution ispermanent.

Alcohol in solution of poly(vinylpyrrolidone) is selected from the groupof ethanol, 1-propanol, 2-propanol or their mixtures.

In advantageous embodiment the poly(vinylpyrrolidone) has an averagemolecular weight within 1000000-1500000 g/mol and its weightconcentration in solution is within the range from 4 to 9%, while themost preferred seems to be the poly(vinylpyrrolidone) having averagemolecular weight of 1300000 g/mol.

Alkoxide of metal is preferably selected from the group of titaniumtetrabutoxide, titanium tetraisopropoxide, aluminium tri-sec-butoxide,aluminiumtriisopropoxide or zirconium tetraisopropoxide.

Alkoxide of semi-metal is preferably tetraethoxysilane or boriumtriethoxide.

To achieve a good stabilisation of alkoxide solution it is preferred ifthe ratio of alkoxide and chelating agent in solution is within 1:0.8 to1:2.2.

For the electrostatic spinning itself, the alkoxide solution inelectrostatic field is to be found on surface of active zone of thespinning mean of a spinning electrode.

At the same time it is preferred, if the alkoxide solution is intoelectrostatic field for spinning transported by surface of the spinningelectrode, which is preferably formed of rotating spinning electrode ofan oblong shape, which extends by a section of its circumference intothe solution being subjected to spinning.

DESCRIPTION OF THE DRAWING

The drawing represents in the FIG. 1 produced TiO₂ nanofibres, and inthe FIG. 2 their XRD spectrum, the FIG. 3 represents Al₂O₃ nanofibresand the FIG. 4 their XRD spectrum, the FIG. 5 represents B₂O₃ nanofibresand the FIG. 6 their XRD spectrum, the FIG. 7 represents ZrO₂ nanofibresand the FIG. 8 their XRD spectrum.

EXAMPLES OF EMBODIMENT

Spinning solution for production of inorganic nanofibres, especially ofTiO₂, SiO₂, Al₂O₃, ZrO₂ and B₂O₃, by means of electrostatic spinningcontains as a source of inorganic basis an alkoxide of respective metal,semi-metal or non-metal, which is dissolved in a suitable solvent, e.g.in ethanol, 1-propanol or 2-propanol. To stabilise the solution ofalkoxide, especially to prevent its hydrolysis, an addition of chelatingagent as stabiliser is necessary. The most suitable chelating agent is6-diketone, e.g. acetylaceton. Molecular ratio between alkoxide andchelating agent should be within the range from 1:0.8 to 1:2.2. Toimprove the spinning ability of the solution also supporting polymer isadded into it, which may be represented by e.g. poly(vinylpyrrolidone)having molecular weight of 1300000 g/mol or viscosity number K-90, whileits weight concentration towards a total weight of solution is from 4 to9% by weight.

The process of electrostatic spinning depends on concentration, or moreprecisely on viscosity, surface tension and other parameters of alkoxidesolution. The exact composition of alkoxide solution depends ontemperature and humidity of environment and parameters of electrostaticspinning, such as rotation and type of electrode, distance betweenelectrodes and applied voltage.

In particular example of embodiment for production of SiO₂ nanofibresfor production of solution a mixture of 250 g of ethanol and 39 g ofacetylacetone was used, in which there was carefully dissolved 100 g oftetraethoxysilane. After homogenisation the obtained solution wascarefully mixed with solution of 35.2 g poly(vinylpyrrolidone) havingmolecular weight of 1300000 g/mol in 747.9 g of ethanol. After followinghomogenisation the resultant solution was acidified with concentratedhydrochloric acid.

The resultant solution of tetraethoxysilane was used for production ofSiO₂ nanofibres by means of electrostatic spinning. There was used adevice for electrostatic spinning of polymer solutions comprising aspinning electrode and against it arranged collecting electrode, whilethe spinning electrode comprised rotatably mounted spinning meanextending by a section of its circumference into a solution oftetraethoxysilane being present in a reservoir. The rotating spinningmean, thanks to its rotation, carried out the solution oftetraethoxysilane into a electrostatic field induced between thespinning electrode and the collecting electrode, while a portion ofsurface of rotating spinning mean being positioned against thecollecting electrode represents an active spinning zone of the spinningmean. During spinning the solution of tetraethoxysilane was present inelectrostatic field on surface of active spinning zone of the spinningmean of the spinning electrode. Rotating spinning mean may be construede.g. according to the CZ patent 294274 or according to the CZ PV2006-545 or CZ PV 2007-485. At concrete spinning of solution oftetraethoxysilane described above a portion of solution, about 125 ml,was poured into storing vessel and this was equipped with a spinningrotating cylindrical electrode. The vessel with the electrode waspositioned into a device for production of nanofibres throughelectrostatic spinning. As a substrate material any fabric, foil, etc.,may be used. The obtained organic-inorganic nanofibrous layer comprisedthe nanofibres having thickness of 30-1000 nm.

The nanofibrous organic-inorganic layer was consequently calcinated in afurnace in air atmosphere at temperature from 600 to 800° C. atproduction of pure amorphous SiO₂ nanofibres.

Similarly, the following solutions of alkoxides were subject toelectrostatic spinning.

For production of TiO₂ nanofibres for preparation of solution themixture of 250 g of ethanol and 29.4 g of acetylacetone was used, inwhich 100 g of titanium tetrabutoxide was dissolved. Afterhomogenisation the obtained solution was carefully mixed with solutionof 35.2 g of poly(vinylpyrrolidone) having molecular weight of 1300000g/mol in 758.8 g of ethanol and after then acidified with concentratedhydrochloric acid. The resultant solution was subject to spinningthrough electrostatic spinning. The nanofibrous organic-inorganic layerwas calcinated in furnace in air atmosphere at the temperature of 500°C. The crystallic form (structure) of resultant TiO₂ of inorganicnanofibres was purely of anatase.

In further example of embodiment for production of Li₄Ti₅O₁₂ nanofibresfor preparation of solution the mixture of 250 g of ethanol and 29.4 gof acetylacetone was used, in which 100 g of titanium tetrabutoxide wasdissolved. After homogenisation the obtained solution was carefullymixed with solution of 35.2 g of poly(vinylpyrrolidone) having molecularweight of 1300000 g/mol and 24 g of dihydrate of lithium acetate in758.8 g of ethanol. The resultant solution was subject to electrostaticspinning. The nanofibrous organic-inorganic layer was calcinated infurnace in air atmosphere at the temperature of 750° C. The resultantinorganic fibres showed the phase of Li₄Ti₅O₁₂ with addition of anataseand rutile less than 5%.

In another example of embodiment for production of solution of TiO₂nanofibres the mixture of 250 g of 2-propanol and 29.4 g ofacetylacetone was used, in which 100 g of titanium tetrabutoxide wasdissolved. After homogenisation the obtained solution was mixed withsolution of 35.2 g of poly(vinylpyrrolidone) having molecular weight of1300000 g/mol in 758.8 g of ethanol, and after then acidified withconcentrated hydrochloric acid. The resultant solution was subject tospinning through electrostatic spinning. The nanofibrous layer wascalcinated at the temperature of 700° C. Crystallic form of resultantnanofibres was partially of anatase and partially of rutile, that iswitnessed by the picture represented in the FIG. 1 and XRD spectrum ofnanofibres represented in the FIG. 2.

In further exemplary embodiment for production of solution forproduction of TiO₂ nanofibres the mixture of 250 g of 1-propanol and29.4 g of acetylacetone acidified with 0.3 g of phosphoric acid wasused. In the given mixture 100 g of titanium tetrabutoxide wasdissolved. After homogenisation the obtained solution was mixed withsolution of 35.2 g poly(vinylpyrrolidone) having molecular weight of1300000 g/mol in 758.8 g of ethanol. The resultant Solution was subjectto spinning through electrostatic spinning. The nanofibrous layer wascalcinated at the temperature of 500° C. Crystallic form of resultantinorganic TiO₂ nanofibres was purely of anatase.

At further example of embodiment for production of solution forproduction of TiO₂ nanofibres the mixture of 250 g of ethanol and 58.8 gof acetylacetone was used, in which 100 g of titanium tetrabutoxide wasdissolved. After homogenisation the obtained solution was mixed withsolution of 35.2 g of poly(vinylpyrrolidone) having molecular weight of1300000 g/mol in 729.4 g of ethanol and after then acidified withconcentrated hydrochloric acid. The resultant solution was subject tospinning through electrostatic spinning. The nanofibrous layer wascalcinated at the temperature of 700° C. Crystallic form of resultantTiO₂ nanofibres was partially of anatase and partially of rutile.

For production of solution for production of TiO₂ nanofibres accordingto further example of embodiment the mixture of 150 g ethanol and 29.4 gof acetylacetone was used, in which 100 g of titanium tetrabutoxide wasdissolved. After homogenisation the obtained solution was mixed withsolution of 35.2 g of poly(vinylpyrrolidone) having molecular weight of1300000 g/mol in 272.1 g of ethanol, and after then acidified withconcentrated hydrochloric acid. The resultant solution was subject tospinning through electrostatic spinning. The nanofibrous layer wascalcinated at the temperature of 500° C. Crystallic form of resultantTiO₂ nanofibres was purely of anatase.

In another embodiment for production of solution for production of TiO₂nanofibres the mixture of 250 g ethanol and 35.2 g of acetylacetone wasused, in which 100 g of titanium tetraisopropoxide was dissolved. Afterhomogenisation the obtained solution was mixed with solution of 42.2 gof poly(vinylpyrrolidone) having molecular weight of 1300000 g/mol in977.7 g of ethanol and after then acidified with concentratedhydrochloric acid. The resultant solution was subject to spinningthrough electrostatic spinning. The nanofibrous layer was calcinated atthe temperature of 500° C. Crystallic form of resultant TiO₂ nanofibreswas purely of anatase.

For production of Al₂O₃ nanofibres the mixture of 500 g of 2-propanoland of 40.7 g of acetylacetone was used, in which 100 g of aluminiumtri-sec-butoxide was dissolved. After homogenisation the obtainedsolution was mixed with solution of 62.1 g of poly(vinylpyrrolidone)having molecular weight of 1300000 g/mol in 1366.9 g of ethanol andafter then acidified with concentrated hydrochloric acid. The resultantsolution was subject to spinning through electrostatic spinning. Thenanofibrous layer was calcinated at the temperature of 700° C. Theresultant inorganic fibres showed a pure □-Al₂O₃ crystallic structure,which is witnessed by the picture represented in the FIG. 3 and XRDspectrum represented in the FIG. 4.

In another embodiment for production of solution for production of Al₂O₃nanofibres the mixture of 350 g of 2-propanol and 40.7 g ofacetylacetone was used, in which 100 g of aluminium tri-sec-butoxide wasdissolved. After homogenisation the obtained solution was mixed withsolution of 62.1 g of poly(vinylpyrrolidone) having molecular weight of1300000 g/mol in 827 g of ethanol and after then acidified withconcentrated hydrochloric acid. The nanofibrous layer was calcinated atthe temperature of 800° C. The resultant inorganic fibres showed a pure□-Al₂O₃ crystallic structure.

In another embodiment for production of solution for production of Al₂O₃nanofibres the mixture of 500 g of 2-propanol and 49 g of acetylacetonewas used, in which 100 g of aluminium triisopropoxide was dissolved.After homogenisation the obtained solution was mixed with solution of74.9 g of poly(vinylpyrrolidone) having molecular weight of 1300000g/mol in 1772.2 g of ethanol and after then acidified with concentratedhydrochloric acid. The resultant solution was subject to spinningthrough electrostatic spinning. The nanofibrous layer was calcinated atthe temperature of 700° C. The resultant inorganic fibres showed a pure□-Al₂O₃ crystallic structure.

For production of B₂O₃ nanofibres the mixture of 500 g of ethanol and68.6 g of acetylacetone was used, in which 100 g of borium triethoxidewas dissolved. After homogenisation the obtained solution was mixed withsolution of 71.5 g of poly(vinylpyrrolidone) having molecular weight of1300000 g/mol in 1644.3 g of ethanol and after then acidified withconcentrated hydrochloric acid. The resultant solution was subject tospinning through electrostatic spinning.

The nanofibrous layer was calcinated at the temperature of 500° C. Theresultant inorganic fibres showed B₂O₃ crystallic structure withamorphous addition, which is witnessed by the picture represented in theFIG. 5 and XRD spectrum represented in the FIG. 6.

For production of ZrO₂ nanofibres the mixture of 500 g of ethanol and30.6 g of acetylacetone was used, into which the solution of 142.9 g ofzirconium tetraisopropoxide in 1-propanol was added. Afterhomogenisation the obtained solution was mixed with solution of 56.4 gof poly(vinylpyrrolidone) having molecular weight of 1300000 g/mol in1193.8 g of ethanol and after then acidified with concentratedhydrochloric acid. The resultant solution was subject to spinningthrough electrostatic spinning. The nanofibrous layer was calcinated atthe temperature of 700° C. The resultant inorganic fibres showed ZrO₂mixture of monoclinic and tetragonal crystallic structure, which iswitnessed by the picture represented in the FIG. 7 and XRD spectrumrepresented in the FIG. 8.

In all cases the long-term continual spinning process was achieved andthickness of produced nanofibres was from 30 to 1000 nm.

Production of nanofibres from the above mentioned solutions of alkoxidesis not limited only to the described electrostatic spinning device withrotating spinning electrode, but it is possible to use also other typesof spinning electrodes, at which the solution of alkoxide inelectrostatic field for spinning is to be found on surface of activespinning zone of a spinning mean of a spinning electrode. Spinning ofalkoxide solution runs successfully also on wire spinning electrodesaccording to the CZ PV 2007-485, at which the active spinning zone ofthe wire has during the spinning process a stable position towards thecollecting electrode and alkoxide solution is to the active spinningzone of the wire supplied either by applying or by movement of the wirein direction of its length. In this case, the solution of alkoxide inelectrostatic field for spinning is to be found on surface of activezone of the wire of spinning mean. The described solutions of alkoxidescan of course be used also for discontinuous production of nanofibres atusage of nozzle or needle as a spinning electrode.

INDUSTRIAL APPLICABILITY

The mentioned method for production of nanofibres ensures a sufficientstability of the solution being subject to spinning for entire period ofspinning, and it is a key aspect for continuous production of inorganicnanofibres. Application of layers of inorganic nanofibres is importantin many technical fields and industry, e.g. for production of compositematerials, catalysts and electrochemical active elements.

1. A method for production of inorganic nanofibres through electrostaticspinning of solution, which comprises alkoxide of metal or of semi-metalor of non-metal dissolved in a solvent system on basis of alcohol,wherein the solution is stabilised by acethylacetone, which preventshydrolysis of alkoxide, and after homogenisation it is mixed withsolution of poly(vinylpyrrolidone) in an alcohol, after then theresultant solution is brought into electrostatic field, in which theelectrostatic spinning is running continually, the result of which isproduction of organic-inorganic nanofibres, which are after thencalcinated outside the spinning device in air atmosphere at thetemperature from 500 ° C. to 1300° C.
 2. The method according to claim1, wherein, to increase the electrical conductivity of the solution, aconcentrated acid is added into the solution.
 3. The method according toclaim 2, the wherein the acid is selected from the group of hydrochloricacid, nitric acid, phosphoric acid.
 4. (canceled)
 5. The methodaccording to claim 1, wherein the alcohol in solution ofpoly(vinylpyrrolidone) is selected from the group of ethanol,1-propanol, 2-propanol or their mixtures.
 6. The method according toclaim 1, wherein the poly(vinylpyrrolidone) has an average molecularweight within the range of 1000000-1500000 g/mol and its weightconcentration in the solution is within the range from 4 to 9%.
 7. Themethod according to claim 6, wherein the poly(vinylpyrrolidone) hasaverage molecular weight of 1300000 g/mol.
 8. The method according toclaim 1, wherein the poly(vinylpyrrolidone) has viscosity number Kwithin the range from K-70 to K-95 and its concentration in solution isin the range from 4 to 9%.
 9. The method according to claim 8, whereinthe poly(vinylpyrrolidone) has viscosity number K-90.
 10. The methodaccording to claim 1, wherein the alkoxide of metal is selected from thegroup of titanium tetrabutoxide, titanium tetraisopropoxide, aluminiumtri-sec-butoxide, aluminium triisopropoxide, zirconiumtetraisopropoxide.
 11. The method according to claim 1, wherein thealkoxide of semi-metal is selected from the group of tetraethoxysilane,borium triethoxide.
 12. The method according to claim 1, wherein themolecular ratio of alkoxide and chelating agent in solution is from1:0.8 to 1:2.2.
 13. The method according to claim 1, wherein thealkoxide solution in electrostatic field for spinning is to be found onsurface of active spinning zone of the spinning mean of the spinningelectrode.
 14. The method according to claim 12, wherein the solution ofalkoxide is transported into electrostatic field for spinning by surfaceof the spinning electrode.
 15. The method according to claim 12, whereinthe spinning electrode is formed of rotating spinning electrode of anoblong shape, which extends by a section of its circumference into thespinning solution.